Embodiments according to the invention relate to a loading state determiner for determining a loading state of an electric power source comprising an internal resistance. Further embodiments according to the invention relate to a load assembly. Further embodiments according to the invention relate to a power supply assembly. Further embodiments according to the invention relate to a method for determining a loading state of an electric power source comprising an internal resistance.
Generally, it can be said that embodiments according to the invention relate to an electronic interface for energy harvesters.
Electric generators are used in many cases for obtaining electric energy based on another form of energy, e.g., mechanical energy, a temperature gradient, chemically stored energy or radiation energy. For an optimum operation of generators, it is desirable to have load matching. This matching is desirable (or in some cases even necessitated) since the generated voltage of a generator (or generally: a power source) can only be tapped at its terminals (i.e., at the sources of generator, or generally the terminals of the power source) via its frequently large internal resistance (or generally source impedance). Typically, for achieving this matching or load matching, a circuit, e.g., a power converter or voltage converter is connected to the generator, which is to load the generator such that the same outputs maximum possible power due to optimum loading.
In the considered generators which are typical for energy harvesting, the load current across the internal resistance (or the source impedance) causes a voltage drop. Thus, the source voltage (e.g., the terminal voltage at the externally accessible terminals of the generator) is reduced. Only with a specific combination of load current and internal resistance (or source impedance) or at a specific terminal voltage, power output is at a maximum.
Load matching is generally achieved when the load impedance (i.e., the ratio between voltage and current at the load) has the value of the complex conjugate source impedance (impedance of the generator). Thus, the load matching is frequently referred to as impedance matching.
Thus, typically, load matching or impedance matching is combined with the fact that at this matched load, the terminal voltage of the generator corresponds to half its instantaneous no-load voltage or open-circuit voltage. Since generators are, in particular in energy-harvesting applications, generally excited by external events—for example vibrations and impacts—the time curve of the resulting no-load voltage is not known.
Considering this, different concepts have been developed for operating an electric generator as efficiently as possible. Here, the superior goal is mostly the operation of a source (e.g., a generator) at the optimum operating point or load point, such that maximum output power is available.
Many conventional systems try to achieve this by gradient-based control algorithms combined with a repeated or permanent power measurement. For details in this regard, reference is made, for example, to U.S. Pat. Nos. 5,867,011, 7,053,506 and U.S. Pat. No. 6,844,739, each describing a gradient method and power measurement. This power determination is performed by simultaneous current and voltage measurement, wherein the determined values are subsequently multiplied.
Other approaches try to maximize the output power via specific assumptions with a complex control algorithm. Details in this regard are described, for example in the publication “Optimized piezoelectric energy-harvesting circuit using step-down converter in discontinuous conduction mode” by G. K. Ottmann, H. F. Hofmann and G. A. Lesieutre (published in: IEEE Trans. Power Electron., vol. 18, pp. 696, March 2003) and in the publication “Buck-boost converter for sensorless power optimization of piezoelectric energy harvester,” by E. Lefeuvre, D. Audigier and D. Guyomar published in: IEEE Trans. Power Electron, vol, 22, pp. 2018, September 2007).
However, these approaches are very complex, whereby in many cases their high internal power consumption has a negative effect.
Apart from this, there are several methods for indirect measurement of no-load voltage from other fields of application, such as battery technology:    1. Measurement in unloaded intervals (see, for example, U.S. Pat. No. 7,557,540 B2):            The voltage is measured in phases where the generator (or the source) is unloaded.        
This takes place in loading intervals or at the beginning of the operation. Here, it is assumed that the no-load voltage of the generator does not change significantly. In summary, it can be said that in the concept according to U.S. Pat. No. 7,557,540 B1, measurement does not take place under load.    2. Auxiliary generator in permanent no-load operation for measuring the no-load voltage (cf. DE 199 04 561):            In addition to the active or used loaded generator, a second generator is operated which is similar to the loaded generator. This second generator is operated in no-load operation, and a no-load voltage of the loaded generator is determined by voltage measurement. Thus, all in all, DE 199 04 561 describes the usage of an unloaded “auxiliary module”.            3. Test measurements with different loads (see, for example, EP 100 3234 A1 and U.S. Pat. No. 6,737,831 B2):            Known test loads are connected to the generator. By comparing the clamp voltages or terminal voltages, the current no-load voltage can be recalculated. Thus, EP 100 3234 A1 describes a test measurement with different test loads. U.S. Pat. No. 6,737,831 B2 describes current injection and voltage difference measurement.        
Considering the conventional concepts for determining the no-load voltage of a generator, it is the object of the present invention to provide a concept for determining loading state or a no-load voltage of a power source that can be implemented with little effort and still provides meaningful information regarding the load state.